Compound transmissions of the range type are well known in the prior art. Such transmissions typically comprise a multiple speed main transmission section connected in series with a range type auxiliary section wherein the range step is commonly greater than the total ratio coverage of the main transmission section.
In automated compound transmissions, the main section is typically shifted by means of an automated actuator responsive to an electronic control unit. The electronic control unit may be integrated into a control unit which operates a plurality of vehicle systems, such as the vehicle engine and the vehicle transmission, or may be a discrete and purpose-specific transmission electronic control unit (“TECU”). The control unit will be generically identified herein as a TECU. The automatic actuator of the main section may be an electric X-Y shifter of the type well known in the art, and described in U.S. Pat. No. 4,873,881, which is hereby incorporated by reference. The automatic main section actuator may alternatively be a pneumatically operated mechanism that is also well known in the art. An automatic range section actuator is responsive to control signals from the TECU. An exemplary range section actuator is shown in U.S. Pat. No. 7,861,612 which is hereby incorporated by reference. The actuator described therein is a pneumatic actuator responsive to electrical signals. Although the source of electrical signals described in U.S. Pat. No. 7,861,612 is an operator controlled switch, the range section actuator could alternatively be responsive to an electrical signal from the TECU. Yet alternatively, the range section actuator could be responsive to a switch controlled by the TECU. The precise mechanisms and configurations thereof used to shift the main transmission section and the range transmission section is not intended to be limiting to scope of application of the present invention.
A common arrangement for a transmission has a plurality of gear ratios available for selection in the main section perhaps five for example, and two gear ratios, characterized as “High” and “Low” provided by the range section. The High range is commonly characterized as “direct” in which the output member of the range section rotates as a unit with the input member. With the range section in the Low range, the output member rotates at a lower speed than the input member, and provides a torque-multiplying effect.
The particular concern addressed by this invention relates to coordinating shifting of the main section and the range section and ensuring engagement of the range section. More specifically, it is intended to facilitate shifting the range section from High to Low in an off-throttle condition, as might be desirable to achieve engine braking when operating a vehicle on a downhill grade. A number of factors relating to the interplay of the mechanical components can contribute to making it difficult to complete such a shift.
It is desired to provide a control system which facilitates the completion of off-throttle range shifts from High into Low.